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A black hole's event horizon is the closed surface-shape surrounding the black hole consisting of a border from within which electromagnetic radiation (EMR) cannot escape. Current physics says there cannot be a way to find out what is within the event horizon other than the black hole's mass, rotation, and charge. The event horizon can be thought of as the outer surface of the black hole, a useful presumption for some approximate models, but the weirdness of black holes results in caveats.
For the simplest black hole (no rotation or charge), the event horizon is a spherical shell-shape located at the Schwarzschild radius (2GM/c²) from the black hole's center. Any black hole rotation makes it "wider" (oblate, as does the rotation of a planet or star), but likely doesn't change the order-of-magnitude.